Add Intervals.icu as a sync target

[sturimcode]

Adds Intervals.icu as a fourth target. Each day's wellness record gets the weight and body fat from that day's newest weigh-in.

• eufy-sync --setup-intervals asks for an API key, checks it against your athlete profile before saving, and finds the athlete ID on its own. The key goes into the credential store; config.yaml holds only the athlete ID. --disconnect-intervals removes the target and its key. First-run setup offers it alongside the other targets.
• Sync works per day. A weight-only reading and its full reading count as one weigh-in, the newest weigh-in of the day wins whatever order Eufy returns them in, and a day is sent again only when its values change. A weigh-in's day is fixed when eufy-sync first sees it, so a later timezone change doesn't move it. Failed days use the existing retry queue.
• Errors: a rejected key stops the target and names --setup-intervals; a rate limit waits for the next run; server and network errors are retried.
• --status, --doctor, notifications, the README, and the command reference cover the new target.

873 tests pass locally. Garmin, Strava, and Zwift behaviour is unchanged.

[sturimcode]

Parking this until there's demand for an Intervals.icu target. The branch stays, so it can be reopened if anyone asks for it.